With the need for improvement in data communication speed, the access scheme for mobile phone systems, etc., is transitioning to HSDPA/HSUPA (High Speed Downlink/Uplink Packet Access). With the aim of further increasing speed and capacity and reducing delay, LTE (Long-Term Evolution) standards development is underway for the next generation scheme.
In LTE, while the downlink employs Orthogonal Frequency Division Multiple Access (OFDMA) which is similar to Worldwide Interoperability for Microwave Access (WiMAX), the uplink employs Single Carrier Frequency Division Multiple Access (SC-FDMA) in consideration of terminal power saving.
FIG. 11 shows an example of processes by the SC-FDMA scheme. User terminals 100a, 100b, and 100c performing transmission by the SC-FDMA scheme each include a DFT unit 101, a zero signal inserting unit 102, an IFFT unit 103, a parallel-to-serial converting unit 104, a CP inserting unit 105, and an FE processing unit 106.
The DFT unit 101 transforms transmitted QAM signals (QAM modulated signals) in which information bits are QAM mapped, into frequency domain signals by an N1-point DFT (Discrete Fourier Transform). Note that N1 indicates the number of transmitted QAM signals transmitted by SC-FDMA symbols, and varies between the user terminals 100a, 100b, and 100c. 
Thereafter, the zero signal inserting unit 102 inserts zero signals in bands on a frequency axis in which the user terminal does not transmit data (bands used by the other user terminals).
Then, the IFFT unit 103 transforms the frequency domain signals having the zero signals inserted therein, into time domain signals by an N-point IFFT (Inverse Fast Fourier Transform).
The time domain signals obtained by the IFFT unit 103 are converted into a serial signal by the parallel-to-serial converting unit 104, and then, CPs (Cyclic Prefixes) are appended to the serial signal by the CP inserting unit 105. The serial signal is further subjected to an FE (Front-End) process (D/A conversion, frequency conversion, etc.), and the processed signal is transmitted from an antenna 107 of the user terminal.
A base station apparatus 200 performing reception by the SC-FDMA scheme can simultaneously receive signals from a plurality of users (terminals) by frequency division multiplexing. The base station apparatus 200 has a plurality of receiver circuits 200a, 200b, and 200c provided for respective users, so that the base station apparatus 200 can support simultaneous access from the plurality of users.
Each of the receiver circuits 200a, 200b, and 200c in the base station apparatus 200 includes an FE processing unit 202, a CP removing unit 203, a serial-to-parallel converting unit 204, an FFT unit 205, a zero signal removing unit 206, a transmission path estimating unit 207, a reference signal generating unit 208, a weight computing unit 209, an equalizing unit 210, and an IDFT unit 211.
In the base station apparatus 200, the FE processing unit 202 performs an FE process (frequency conversion, A/D conversion, etc.) on a signal received by an antenna 201. Subsequently, the CP removing unit 203 performs CP removal.
The received signal from which CPs have been removed are converted into parallel signals by the serial-to-parallel converting unit 204. Thereafter, the parallel signals are subjected to an N-point FFT by the FFT unit 205 and are thereby transformed into frequency domain signals.
Furthermore, the zero signal removing unit 206 removes zero signals inserted by a user terminal, the transmitter side, from the frequency domain signals. Note that the bands (subcarriers) in which zero signals are inserted on the transmitter side are identified based on user information indicating frequency bands allocated to the respective user terminals by the base station apparatus.
In addition, the transmission path estimating unit 207 estimates transmission path characteristics from reference signals (known signals) included in the frequency domain signals, and reference signals generated by the reference signal generating unit 208. Then, the weight computing unit 209 computes weights based on the estimated transmission path characteristics.
The equalizing unit 210 performs equalization by adding up the frequency domain signals (received signals) from which zero signals have been removed, with the weights. Then, the IDFT unit 211 performs an IDFT on the signals outputted from the equalizing unit 210, and outputs QAM demodulated signals.
In the base station apparatus 200 performing reception by the SC-FDMA scheme, since each of the receiver circuits outputs QAM demodulated signals for each user, such a number of the receiver circuits are required that corresponds to the number of users allocatable at a time. Therefore, a communication apparatus performing reception by the SC-FDMA scheme has a problem of an increase in its circuit size.
Here, Patent Document 1 discloses a technique in which, in order to reduce the amount of processing of a base station apparatus performing reception by the SC-FDMA scheme, the base station apparatus detects received signal timings from users and makes an adjustment such that the timings coincide, whereby an FFT by the base station apparatus, the receiver side, is performed for all of the users at once.